The present invention relates to mechanical drive devices, and, more specifically, to drive devices that will limit the torque being delivered from the device.
Many mechanical devices are used to deliver a large amount of torque to a screw, bolt, nut, or other similar device or object. Even though there is a large amount of torque being delivered, in many situations, it is still desirous to control the precise amount of torque being delivered. For instance, too much torque may strip the object that is being driven, which would lead to the object becoming ineffective, such as stripping a bolt or screw. This is especially important in medical operations and procedures, where precision is critical, especially when working with spinal and skeletal structures and related devices. Thus, drivers have been developed to limit the amount of torque delivered to the driven object or device.
Likewise, improperly calibrated or tensioned devices can also do damage if too much torque is delivered or, conversely, will be ineffective if the tension in the device is too low. Such is the case when working parts of the device become worn, or do not hold a consistent torque after several uses, which can occur in the prior art.
Generally, such torque limiting drivers use ball bearings placed between two clutch plates or between a cam plate and a drive plate. The bearings assist the plates in sliding against one another within a grooved slot formed by the two plates, with the slot having varying depths. As torque is increased with the driver, the ball bearings will slide within the groove. When the torque reaches a maximum, the bearings will be forced into a resting area that prevents the two plates from providing any further torque to the driven object.
Other arrangements include pawls or ramped arrangements. As the torque is increased on the driver, the bearing will move up the ramp until it reaches the top of the ramp, whereby further torque delivery will cause the bearing to move to a resting area, as in the arrangements above.
While these devices limit the amount of torque being delivered, these drivers can take a lot of abuse, especially on the bearings themselves. Especially with medical applications, the amount of torque needed to be delivered can be several hundreds of pounds of pressure. When these devices trigger a torque cut-off when the maximum torque is reached, the two plates will sandwich the bearings, exerting a large amount of pressure on the bearings. This smashing action of the bearings can cause damage to the bearings, which results in the effectiveness of the driver being diminished. This is not desirous for equipment requiring a high-level of precision, especially when the equipment can be rather expensive.
Examples of such drivers may be found in Leonard, et al., U.S. Pat. No. 3,942,337 and Girguis, U.S. Pat. No. 5,035,311. While these devices may effectively limit the torque delivered by the disclosed device, they have a tendency to deliver unneeded forced on the rolling members located between the first and the second clutch members, which limits the effectiveness of the device and, also, the consistency of the torque limit being delivered by the device.
Thus, it would be advantageous to design a torque limiting driver that would minimize damage to the moving parts of the driver.